1. Field of the Invention
The present invention relates to laser velocity interferometry, specifically to an improved laser velocity interferometer which simultaneously measures large changes in velocity of multiple points on a material surface.
2. Discussion of Prior Art
Shock experiments involving projectile impacts or explosive detonations have been used for several decades to determine material properties under dynamic conditions and at extremely high pressures.
One of the most valuable instrumentation techniques in shock experiments has been laser velocity interferometry, in which laser light is focused at a location of measurement on the specimen's surface. Some of the reflected light is collected, and, as the surface moves during a shock experiment, the Doppler shift of the reflected light is measured in an interferometer. The continuous measurement of the Doppler shift results in a continuous velocity history of the location of measurement on the shocked specimen, from which, together with other information, the specimen material properties are calculated.
A review of the field of laser velocity interferometry can be found in the paper by L. M. Barker, "Velocity Interferometry for Time-Resolved High-Velocity Measurements," which appears in Proceedings of SPIE 27th Annual International Technical Symposium and Instrument Display, San Diego, Calif., Aug. 21-26, 1983.
The most common form of laser velocity interferometer has been the so-called Velocity Interferometer System for Any Reflector (VISAR), which was originally developed at Sandia National Laboratories by L. M. Barker and R. E. Hollenbach in the early 1970s. Our first technical paper on the VISAR was L. M. Barker and R. E. Hollenbach, "Laser Interferometer for Measuring High Velocities of Any Reflecting Surface," Journal of Applied Physics, Vol. 43, No. 11, pp 4669-4675, November, 1972.
The term VISAR generally includes (a) the laser velocity interferometer which produces good fringe contrast even when illuminated by light from a diffusely reflecting surface, (b) any optical elements necessary to make the input signal light into a beam suitable to traverse the optics of the interferometer, (c) any optics both within and outside of the interferometer which are necessary for producing fringes in quadrature, (d) any optics which are involved in producing the required delay time in one of the light paths of the interferometer, (e) any optics to direct the output signal light to light detectors, (f) the light detectors themselves, and (g) any incidental optical elements to direct, shape, filter, or adjust the intensity of the light beams which traverse or interact with the aforementioned components. Any optical mounts, frames, enclosures, adjusting devices, etc., associated with the aforementioned components are also generally considered to be included in the term VISAR. In addition, the optical elements which guide the laser light to the location of measurement on the specimen, and which gather reflected light from the location of measurement and guide it to the VISAR interferometer are often included.
The VISAR works by using a beamsplitter to split the incident light beam, which is composed of light reflected from a location of measurement on a specimen, into the two light paths (legs) of an interferometer. The legs of the interferometer have unequal light travel times before the two split-off light beams are recombined, i.e., one of the light beams is delayed slightly with respect to the other. However, in spite of the delay time they meet the criteria for forming high-contrast fringe patterns, even when the interferometer is illuminated by light from a diffusely reflecting surface.
The delay time in the VISAR interferometer causes it to produce a shift in interference fringes whenever the wavelength of the light beam through the interferometer changes. Thus, if the surface acquires a velocity, the wavelength of the reflected light changes by the Doppler effect, and the VISAR interferometer interference fringe position shifts. The amount of the shift is proportional to the change in the reflecting surface velocity, and also to the delay time in the VISAR interferometer.
The specimen velocity change which causes a fringe shift of one fringe is called the VISAR's Velocity-Per-Fringe (VPF) constant. VISARs are usually made to allow for changing the VPF by adding or subtracting delay etalons in the delay leg of the VISAR. The VPF of such multi-etalon VISARs can be changed to best fit the needs of a particular experiment.
When a VISAR interferometer is properly aligned, the output signal beams normally show only the central "bull's eye" of the interferometer's fringe pattern, where only a small part of a fringe is visible at any one time, and a fringe shift of one fringe appears as one complete cycling of the light intensity.
The light fringes produced by a VISAR in a velocity measurement are normally recorded using light detectors, such as photomultipliers, to change the fringe light intensity variations into voltage variations. Digitizing oscilloscopes may be used to record the voltage variations. The voltage-time data points collected by the oscilloscopes can then be analyzed in a computer program to obtain the velocity vs. time of the measured location during the experiment. Streak cameras have also been used to record the VISAR fringe shifts during an experiment.
VISARs can use polarization coding to obtain sets of fringes approximately 90.degree. out-of-phase with each other. This greatly enhances the accuracy of the data, allowing the fringe count to be determined at any time to about .+-.2% of one fringe, such that a data record containing four fringes can be expected to be accurate to within 1/2% of the peak velocity. The polarization coding also allows one to distinguish acceleration from deceleration.
The original VISARs had these attributes:
(1) Variable sensitivity to fit the experiment, by varying the delay time, PA0 (2) The ability to measure any surface, whether specular or diffusely reflecting, PA0 (3) Polarization coding for accuracy and for distinguishing acceleration from deceleration, PA0 (4) Fringes in proportion to velocity, not displacement, greatly decreasing the frequency response required to acquire the data, as well as decreasing the complexity of the data reduction, PA0 (5) Nanosecond time resolution, PA0 (6) Better than 1% accuracy in most experiments, and PA0 (7) Absence of any perturbation (by the instrumentation) of the velocity being measured.
Improvements to the VISAR A 1976 paper by B. T. Amery, "Wide Range Velocity Interferometer," in Sixth Symposium on Detonation (Office of Naval Research, Dept. of the Navy, Arlington Va., Aug. 24-27, 1976), pp. 673-681, pointed out that the delay etalons in a VISAR interferometer can be replaced by two lenses separated by the sum of their focal lengths. A much wider range of delay times is available with the lens-generated delay leg, which allows for accurate measurements of smaller velocities when long delay times are used. The present invention relates not only to multi-etalon VISARs, but to Amery's lens delay leg VISARs as well.
A very significant improvement to the VISAR was made by W. F. Hemsing in 1978, and published in his paper "Velocity sensing Interferometer (VISAR) Modification," Review of Scientific Instruments, Vol. 50, No. 1, pp 73-78, 1979. Hemsing's improvement, called the "push-pull VISAR," cuts the amount of required laser light by at least 50% without any sacrifice in the signal-to-noise ratio of the instrument by making better use of the light emerging from the VISAR interferometer. In addition, stray non-laser light which may find its way into the signal light beam, such as self-light generated by the experiment, is largely self-canceling. Hemsing's push-pull improvement retains all of the above listed attributes.
Another major VISAR advance which retains all of the above attributes, including the Hemsing improvement, is described in U.S. Pat. No. 5,481,359 to Barker (1996). The patent covers design features which make VISARs smaller, portable, stable, and easy-to-use. Over 30 VISARs based on this patent have already been sold and delivered.
Because of the VISAR's impressive list of attributes, it has become widely recognized as the instrumentation technique of choice in certain applications requiring accurate measurement of large velocity changes.
Nevertheless, the usefulness of VISARs has been limited by the fact that they normally measure the velocity of only one location at a time. Attempts to overcome this limitation have involved devoting more than one VISAR at a time to a specimen, or measuring different locations on a specimen in successive identical experiments. Neither of these approaches is very satisfactory because VISARs, and especially the VISAR interferometers with their delay etalons, are quite expensive, costing at least tens of thousands of dollars each. In addition, the experiments can also be very expensive, and achieving sufficiently good experimental repeatability to measure different locations in successive experiments can be difficult or impossible.
In recent years, "Line VISAR" instrumentation has been developed which in principle measures the velocity at all locations along a straight line on the specimen surface. The technique was described by W. F. Hemsing, A. R. Mathews, R. H. Warnes, M. J. George, and G. R. Whittemore, "VISAR: Line-Imaging Interferometer," in Shock Compression of Condensed Matter, pp. 768-770 (1992), .COPYRGT. Elsevier Science Publishers B. V. It involved using a laser beam and a cylindrical lens to illuminate a line across a specimen surface. Light reflected from the line was focused through the VISAR interferometer such that real images of the line appeared at the entrance slit of a streak camera, which recorded the fringe shifts along the line image. A similar line imaging VISAR with apparently improved optics was reported by K. Baumung, J. Singer, S. V. Razorenov, and A. V. Utkin, "Hydrodynamic Proton Beam-Target Interaction Experiments Using an Improved Line-Imaging Velocimeter," in Shock Compression of Condensed Matter--1995, pp. 1015-1018 (1996), AIP Press, Woodbury, N.Y.
The line-imaging VISARs of both Hemsing, et. al., and Baumung, et. al., rely on streak cameras to record the data, with consequent difficulties such as more crude data reduction techniques and a very small number of data points in time compared to digitizing oscilloscope recordings. Also, data collection is limited to a relatively short straight line on the specimen, whereas data from a larger area, or at least a non-linear one, would often be an advantage.
In the early 1980s, Dr. Datta Dandekar of the U. S Naval Research Laboratories contracted with the University of Arizona Optics Department to implement his ideas for a four-beam VISAR. The design sends four data beams simultaneously through a single VISAR interferometer. The resulting instrument was so difficult to align that it could not be productively used. Finally, its design was extensively modified and converted to optical fiber light transport in 1991-92 by L. M. Barker of Valyn, International. Since then, Dandekar's four-beam VISAR has been a viable velocimeter, capable of measuring the velocities of up to four locations simultaneously, anywhere on a specimen. However, I know of no published documentation of this VISAR.
The main difficulty with the Dandekar four-beam VISAR is its poor fringe quality, caused by its use of a different collimator for each of the light signals reflected from the four locations of measurement on the specimen. Because of the unavoidable physical size of the collimators, the effective size of their combined output beams is quite large, thereby precluding high-quality interferometer fringes without prohibitively large and expensive interferometer optical components.
Other drawbacks have been the difficulty of aligning the instrument's interferometer for optimum fringes, and the complications of changing its sensitivity, or velocity-per-fringe constant. No other laboratory has attempted to duplicate nor improve on the ARL four-beam VISAR design despite its use by Dandekar's group for the past six years. The fact that 50 to 100 new single-beam VISARs have been constructed during this time, four of which were purchased by Dandekar's group, is indicative of the problems of this multi-beam VISAR.